Apparatus for floating-zone melting



'. 8, 1964 T. RUMMEL ETAL APPARATUS FOR FLOATING-ZONE MELTING 2 Sheets-Sheet 2 Filed June 6, 1960 1 Claim. (61. zs 27s Our invention relates to an apparatus for the crucible free zone melting, also called floating-zone melting, of rod-shaped silicon or other semiconductor material. In such methods a narrow zone of liquid semiconductor material is produced by an induction heater coil which is between rod portions whose ends are attached to holders. The induction coil is generally caused to travel along the rod axis, so as to pass the narrow molten zone along substantially the entire length of the rod. This method does not require the use of a crucible since the molten zone is kept floating between the solid portions of the rod, thus avoiding contamination of the semiconductor substance by material from a crucible wall.

During the operation the two rod portions attached to the holders must be shifted toward or away from each other in order to control the thickness of the semiconductor rod during the zone pulling operation. This is required, for example, to produce a rod whose diameter remains constant within a given range.

It is an object of our invention to provide an improved floating-zone apparatus of this kind, in particular with respect to greater reliability of performance, and to eliminate the need for movable electric circuit connections to the induction heater coil.

According to our invention, we keep the induction heater coil stationary and displace the two rod portions simultaneously in the same direction and, substantially, at the same rate relative to the coil.

According to another feature of the invention we obtain such simultaneous displacement by connecting both rod holders to a common drive means.

According to still another feature of the invention, we provide an adjusting device travelling together with the holders and we actuate the adjusting device by the above-mentioned drive so as to vary the mutual axial spacing between the holders. That is, for obtaining a given, especially a constant thickness of the rod, the two holders and hence the rod portions are moved toward or away from each other while both portionstravel along the semiconductor rod, thus causing upsetting or stretching of the molten zone, as may be required for producing the desired diameter.

The invention will be further described with reference to preferred embodiments of apparatus, as illustrated in the drawings in which each of FIGS. 1, 2 and 3 is a partly sectional front view of a different zone pulling apparatus. The same references are used in all illustrations for respectively similar components.

In the apparatus shown in FIG. 1 the semiconductor rod 1, consisting for example of silicon, has its two ends attached to respective holders 1' and 2 and is located in a vessel of metal or quartz containing a protective gas atmosphere or vacuum. At the begining of the zone melting method the rod is first heated to make it conductive, for example by directly passing electric current through the rod, that is through the holders 1' and 2'. Thereafter a narrow zone is melted at one of the two ends. This is done by inductively heating the zone with the aid of a stationary high-frequency coil 14, which is mounted on a standard (not shown) that is entirely within vessel 10. The primary of the induction heater may ice be entirely outside of vessel 10. Thereafter the molten zone is passed along the rod. For this purpose a screw spindle 8 is driven for example from an electric motor 81 whose speed of rotation corresponds to the desired zone-pulling speed. The screw spindle 8 is in threaded engagement with a nut 9 attached to a tubular guide 4. The guide memberd is connected through a second screw spindle 7 with another tubular guide member 3, both guide members being displaceably seated on an upright standard 11 of circular cross section. The two rod portions 1 and 2, joined by the molten zone, are thus displaced in vessel 10 while maintaining a constant axial spacing from each other so that the molten zone is pulled longitudinally through the semiconductor rod. A thickness of the molten zone and of the rod portion recrystallizing therefrom can be controlled by rotating the screw spindle '7 which is in threaded engagement with a nut 5 fastened to the tubular guide 3, and which is connected with the tubular guide 4 through a motor drive 6 mounted on guide 4. As long as spindle 7 is at rest, the mutual spacing of the rod portion remains constant. However, when the drive 6 is actuated to revolve the spindle 7, the spacing between the rod holder 1 and 2' is changed accordingly. The drive 6, preferably comprising an electric motor, can be automatically controlled for example by the anode current of a highfrequency generator which energizes the inductive heater coil 14, this anode current varying in dependence upon the thickness of the semiconductor rod passing through the coil, as is more fully explained in the copending, coassigned application Serial No. 806,174, filed April 13, 1959, now Patent No. 2,913,561.

In the embodiment illustrated in FIG. 2 the molten zone is likewise passed along the rod by revolution of the screw spindle 8. The screw spindle 8 is in threaded engagement with a nut 9 firmly joined with a rail 21 which in turn rigidly interconnects the two guide members 4 and 4'. The guide member 4' is rigidly connected through a screw spindle 7 with the upper guide member 3. Consequently, while the screw spindle 8 is being revolved, the two rod portions 1 and 2 are displaced along the rod axis with a uniform spacing from each other. The desired upsetting or stretching of the molten zone 4, to control the thickness of the zone-melted rod, is carried out by revolving the screw spindle 7, as explained with reference to the embodiment of FIG. 1. i

In the embodiment shown in FIG. 3 the travel of the rod along the stationary high frequency coil 14 as well as the mutual axial displacement of the rod holders 1, 2' is performed with the aid of a single screw spindle 30. The spindle has two threaded portions 7' and 8' which correspond in function to the spindles 7 and 8 in FIGS. 1 and 2. The uniform longitudinal displacement of the two red portions 1 and 2, with constant mutual spacing, is effected by revolving the spindle 30 with the aid of the drive 12. By turning a nut 5' with the aid of a drive 6, the two holders 1' and 2 can be moved toward or away from each other.

In each of the embodiments the rod holders can be rotated about the axis of the rod in the same or opposite directions by means of pulleys 19 and 20.

The new apparatus permits the production of a hyperpure monocrystalline semiconductor rod the diameter of which is constant, or varies along the length of the rod in a predetermined, desired manner. These rods are suitable for fabrication into semiconductor components, for use in transistors, rectifiers and other electronic semiconductor devices.

We claim:

An apparatus for carrying out a crucible-free floatingzone melting of a crystallizable semiconductor rod, comprising a vessel, a fixed annular induction heating coil in said vessel, the rod passing through said. coil, two

vertically coaxial rod holders supportingsaid rod in a v Q1 7 moving said screw spindle means whereby the spacing between the holders is altered to control the diameter of the recrystallized rod. 7

References Cited in the file of this patent UNIT D sTATEs PATENTS Rurnmelet al Nov. 17, 1959 Emeis VFeb. 21, 1961 OTHER REFERENCES Keck et al.: The. Review of Scientific Instruments,

vol. 25,,N0. 4, pp. 331-334, April 1954. 

